The present invention generally relates to a four-cycle engine and, more particularly, to an oil pan structure effective in a four-cycle engine, in which oil lubrication is performed by a forced oil feed method.
In recent years, demands for exhaust emission control and improvement of fuel economy against environment problems have promoted the employment of four-cycle engines to be mounted on vehicles.
However, the four-cycle engine requires an oil pan to be provided in a lower part thereof so as to perform oil lubrication of each part of the engine. Thus, the four-cycle engine has a problem in that the outside dimension thereof is large, as compared with that of a two-cycle engine.
Therefore, it is desired that a four-cycle engine to be mounted on a vehicle, such as a small snowmobile, which has limited storage space, is small and saves space as much as possible.
Hereinafter, oil lubrication to be performed in a conventional four-cycle engine is described.
The oil lubrication to be performed in the conventional four-cycle engine has the following steps. That is, oil fed by, for example, an oil pump to each of parts to be lubricated runs down after the lubrication. Then, the oil is stored in an oil pan provided in a lower part of the engine. Subsequently, the stored oil is sucked by the oil pump, and fed to each of the parts to be lubricated.
Dry sump method and a wet sump method are known as the oil lubrication methods for a four-cycle engine.
The wet sump method is adapted so that all oil is stored in an oil pan provided in a lower part of a crankcase, that the oil is fed from the oil pan by the oil pump to each of the parts to be lubricated in the engine, and that return oil completed the lubrication is stored again in this oil pan.
On the other hand, the dry sump method is adapted so that the oil tank separated from an engine body is provided, that return oil stored in the oil pan upon completion of the lubrication is sucked by an oil pump, and then fed to the oil tank, and that the oil is fed by another oil pump from the oil tank to each of parts to be lubricated.
That is, according to the dry sump method, there is no need for storing all oil in the lower part of the engine. Thus, as compared with the wet sump method, the capacity of the oil pan can be reduced. Consequently, the dry sump method has merit in that the height of the engine can be reduced.
Hitherto, there has been proposed an apparatus configured by modifying an oil pan in such a way as to prevent the oil pan from interfering with other constituent parts arranged in the lower part of a four-cycle engine, so as to save space accommodating the engine in the case of mounting the engine employing the wet sump method on, for instance, a vehicle, such as a small snowmobile, which has limited space for accommodating the engine (see the patent document: Japanese Application Publication Number: 2001-193559 (KOKAI 2001-193559) pages 3 to 4, and FIG. 1).
However, according to the aforementioned conventional art apparatus, the engine can be mounted by modifying the oil pan in such a way as to have a small height, while the capacity of the oil pan itself is unchanged, so that the size of an engine body is substantially unchanged.
Then, the capacity of the oil pan can be reduced by employing the dry sump method as a method for performing oil lubrication in the engine. However, the conventional art apparatus has problems in that when the capacity of the oil pan is reduced, the routing of an oil strainer to be installed in an intake pump (that is, as cavenging pump) becomes complicated, so that the oil strainer is subjected to shape constraints.
Further, because the intake pump feeds oil to the separated oil tank, it is necessary that the capacity of the intake pump is more than that of a supply pump (that is, a feed pump) for supplying oil to each of parts of the engine, which are to be lubricated, and that the oil strainer ensures a section area sufficient to the extent that the oil strainer can be prevented from causing intake resistance.
For example, when the oil strainer is formed from a pipe member, the oil strainer is subjected to pipe-diameter and routing constraints because it is impossible to form the oil strainer into a steeply curved shape. Further, when the oil strainer is formed by resin molding or casting, the apparatus has a problem in that the structure of a stationary part of the oil strainer becomes complicated so as to be prevented from being damaged owing to vibrations thereof.
Moreover, in the case of an engine to be mounted on a snowmobile, an oil filter is disposed in front of the engine owing to maintainability. Therefore, it is difficult to dispose the oil pump at the engine's rear portion in which the placement of an oil gallery is difficult. With such configuration, when the engine is mounted thereon during a state in which the engine is backwardly tilted, it is difficult to ensure the oil passage led to the scavenging pump.